This invention relates to semiconductor devices with high electron mobility utilizing electrons accumulated in the neighborhood of a single heterojunction due to the difference in electron affinity between two different kinds of semiconductors which interleave a single heterojunction therebetween and methods for production thereof. More specifically, this invention relates to an improvement applicable to high electron mobility transistors (hereinafter referred to as "HEMT") which are Field Effect Transistors (FET'S) provided with an undoped or unintentionally doped GaAs layer and an N-type AlGaAs layer interleaving a single functional heterojunction therebetween and allowing electrons with high mobility to be accumulated in the undoped or unintentionally doped GaAs layer in the form of quasi-two-dimensional electron gas appearing contiguous with the single functional heterojunction due to the difference in electron affinity between GaAs and AlGaAs. Thereby, the accumulated electrons in the form of quasi-two-dimensional electron gas and having high mobility, function as the sole carriers or conductive channel of the FET's. Further, this invention relates to an improvement applicable to the methods for production of HEMT's described above.
A HEMT is an active semiconductor device of which the conductive channel consists of electrons accumulated in a quadratic surface contiguous with a single heterojunction interleaved between a layer of a semiconductor having a lower electron affinity (e.g. AlGaAs) and another layer of a semiconductor having a larger electron affinity (e.g. GaAs).
One example of HEMT's is an FET having a layer configuration comprising an N-type AlGaAs layer grown on an undoped GaAs layer further grown on a Cr-doped GaAs substrate having a high resistance. Also comprising the HEMT is a Schottky barrier type gate electrode, and a pair of source and drain electrodes ohmicly contacted with a conductive channel formed by a two-dimensional electron gas accumulated in the undoped GaAs layer in the form of a plane contiguous with a single heterojunction interleaved between the N-type AlGaAs layer and an undoped GaAs layer. The outstanding feature of HEMT's is the extremely high electron mobility availble in the conductive channel or the two-dimensional electron gas at cryogenic temperatures, e.g. 4.degree. K. through 77.degree. K. The thickness of this conductive channel or two-dimensional electron gas is extremely small specifically less than 100 .ANG.. Furthermore, two-dimensional electron gas is accumulated in the undoped GaAs layer in the form of a plane contiguous with the single heterojunction of the N-type AlGaAs layer and an undoped GaAs layer, and has a geometrical position separate from the N-type AlGaAs layer which supplies electrons to the two-dimensional electron gas. Therefore, the mobility of electrons comprising the two-dimensional electron gas is free from the effects of ionized-impurity scattering. On the other hand, it is well-known that ionized-impurity scattering is the major factor preventing the electron mobility from increasing at cryogenic temperatures. As described earlier, since ionized-impurity scattering has no effect on the magnitude of electron mobility of the electrons in the two-dimensional electron gas, the electron mobility therein becomes extremely large at cryogenic temperatures at which ionized-impurity scattering is inherently the major factor preventing electron mobility from increasing. The results of experiments show that the magnitude of this improvement in electron mobility is more than 10.
Since the N-type AlGaAs layer supplies electrons to the two-dimensional electron gas, the N-type AlGaAs layer is depleted to some extent. Therefore, when the thickness and impurity concentration, the magnitude of the Schottky barrier of the gate and the difference in electron affinity between a semiconductor which supplies electrons to the two-dimensional electron gas (N-type AlGaAs in this case) and the other semiconductor in which the two-dimensional electron gas is accumulated (undoped GaAs in this case) are each the proper magnitude, it is possible to entirely deplete the N-type AlGaAs layer. This is because the depletion layer caused by the Schottky barrier gate and the depletion layer caused by the movement of electrons into the two-dimensional electron gas due to the difference in electron affinity between AlGaAs and GaAs in contact with each other under thermal equilibrium. Since the undoped GaAs is non-conductive, the quadratic electron gas functions as the sole channel for the layer configuration described above. Further, when a voltage is applied to the gate electron, the electron concentration of the two-dimensional electron gas can be readily modulated, thereby the FET having the layer configuration described above can have a large magnitude of transfer conductance Gm.
The inventors of this invention have discovered that the magnitude of electron mobility in the two-dimensional electron gas is sizably decreased by heat treatment or annealing applied thereto. They assumed this undesirable decrease in electron mobility is caused by the effect of ionized-impurity scattering of impurities, for example Si, diffused to the semiconductor layer in which the two-dimensional electron gas is accumulated, undoped GaAs in this case, from the semiconductor layer which supplies electrons to the two-dimensional electron gas, N-type AlGaAs in this case, during the heat treatment or annealing process. A known method effective to prevent unexpected diffusion of impurities from occurring during the growing of semiconductor layers is to interleave an undoped semiconductor layer between two layers having a different impurity concentrations. However, it would be impossible to absolutely prevent impurities from diffusing into the undoped GaAs layer from the N-type AlGaAs layer.
In addition, the results of experiments carried out by the inventors of this invention show that not only an N-type impurity such as Si which is assumed to have diffused from the N-type AlGaAs layer but also an impurity which produces a deep level, such as Cr, is contained in the undoped GaAs layer. Since this impurity uniformly spreads throughout the entire 1-.mu.m thickness of the undoped GaAs layer, it would be impossible to prevent this type of impurity diffusion from occurring, unless some other layer configuration is employed.
Further, it is widely recognized that it is not easy to grow a ternary or quaternary semiconductor layer. It is also widely recognized that the molecular beam epitaxy process or the ion beam epitaxy process is most appropriate for the growth of such layers, because it allows production of an abrupt heterojunction, and accurate control of thickness and of doping concentration. However, it is not necessarily easy to accurately control the composition of such a compound semiconductor. Unfortunately, however, control of the composition of AlGaAs and GaAs layers is extremely important in the production of HEMT's, because these compositions are included in the parameters determining the quality of HEMT's.